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ISDN Connection Management
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IP Specifics
IP Address Connect
As stated previously, an IP Address Connect connection is an ISDN connection that is established to a specific
destination router dependent upon the destination IP address contained within IP traffic received from the local
LAN.
This means that the router continuously monitors IP traffic from the local LAN, as all IP routers do, and makes
ISDN connections to partner routers when IP traffic needs to be sent to remote LANs. Once the IP traffic is
passed to the remote LAN and all sessions are closed, the local router will then disconnect the ISDN call and
continue to monitor the local LAN for IP traffic.
The IP Address Connect table consists of IP addresses and associated ISDN numbers of remote partner IP
Routers. Either one or both of the ISDN calls may be used for IP Address Connect use. When one of the ISDN
calls is defined for Auto-Call purposes, the remaining ISDN call may be used for IP Address Connect use.
The combination of Auto-Call and Address Connect allows part of the WAN environment to be established
statically and still allow dynamic connections to other networks depending upon destination IP addresses.
Suspension of TCP/IP Sessions
When Connection Management is enabled, TCP sessions that are established across the ISDN calls are
monitored and maintained in a table. The TCP session table may contain up to 256 entries. The will filter all
traffic for any TCP/IP session over the 256 limit of the table preventing any new TCP sessions from being
established.
While an ISDN call is suspended, if the router observes that one of the devices in the session stops sending
keepalive packets, the router will begin to generate keepalive packets and send them to the device in order to
determine the status of the device. The length of time the router waits before beginning to generate keepalive
packets is definable by the operator.
If the device does not respond to five consecutive keepalive packets sent from the router, the router will
determine that the device has gone away and the router will send a packet to each end of the TCP session to
shut down the session. When the shut down packet is generated for the remote device, the router will re-
activate the suspended ISDN call and transmit the shut down packet to the remote device. Once the remote
router determines that the TCP session is no longer active, the remote router will stop generating and
responding to the keepalive packets for that TCP session.
RIP—Routing Information Protocol
ISDN routers incorporate a 3 second settling time for IP RIP updates. This means that an will wait for three
seconds after an initial change in the network is reported before transmitting that change on to the remaining
routers connected on the Wide Area Network.